Now, lots of apps and players include digital EQ controls, not too mention the psychoacoustic subroutines available from ventures like Dolby, DTS, Bongiovi, and SRS. And Beats has reinvented themselves as a supplier of software bass-boost algorithms for all manner of portable and luggable devices. But not everyone wants to fiddle with a touchscreen to adjust EQ, and not everyone likes the sound of the boost/cut controls baked into their otherwise beloved devices. Well, these devices from FiiO and DigiZoid have you covered, and then some.

The ZO2 in particular offers a dizzying array of tonal flavors, via a 32-postion tone control (I hesitate to call it just a bass-boost, for reasons I'll get to below), while the FiiO is a more workmanlike device, adding a bit of bass boost and extra juice, but also letting you not just use your hard-to-drive cans with your phone, but adding the iOS remote controls that are probably missing from your faves.

I listened to a variety of headphones, ranging from the relatively flat (several interesting pairs of IEMs we have in for review from Logitech and Westone) to the bass-forward (the Sol Republic Tracks, in V8 and V10 driver versions, as well as the flatter v12 version for comparison), along with a longtime low-end fave, the Beats Pro, in order to assess the effect on its own without interference from an already low-end heavy headphone. To see whether these amps provided enough juice to expand one's traveling headphone arsenal, I matched them with the just-barely efficient enough HiFiMan HE-400 and its big brother, the HE-500.